Since
the decline in use of natural stone at the beginning of
the twentieth century most domestic and small commercial
properties in Cornwall and South Devon have been built with
concrete blocks or mass concrete. Until the mid-1950’s concrete
blocks were often locally made, sometimes by individual
builders, and shuttered concrete was mixed on site.
Before the second world war concrete products were rarely
transported more than 20 km. Because concrete was made where
it was needed and as transport was difficult and costly,
aggregates were sought locally. Now all blocks are factory
produced and mass concrete is supplied ready-mixed from
facilities where manufacture is strictly controlled.
In
many parts of Cornwall and South Devon ample supplies of
cheap and often suitably graded aggregate were available
as waste materials from the region’s metalliferous mining
industry. The use of mining and, more particularly, ore
processing wastes as aggregates, is central to the problem
of accelerated concrete degradation in the region. It is
widely called “the mundic block problem”.
Mundic is an old Cornish word for the common sulphide mineral pyrite. Accelerated deterioration is generally associated
with the in situ oxidation of pyrite (and other sulphide
minerals) in mine waste aggregates and sulphuric acid attack
on the cement. Deterioration is occasionally so severe that
concrete becomes structurally unsafe and some properties
have had to be demolished.
In
1994 the Royal Institute of Chartered Surveyors (RICS) published
guidelines for the testing of concrete block and shuttered
mass concrete from properties built prior to the mid-1950's.
This original guidance note has since been up-dated and
amended. The testing regime so established has been designed
to classify the concrete, primarily to satisfy lending institutions
that a property is fit to mortgage on normal terms.
